1. Field of the Invention
This invention relates generally to hydrostatic bearings adapted for use in the head-actuator assembly of a rotating disk data store and more specifically to a zero static-friction rotating-sleeve hydrostatic bearing assembly with an integral rotating-shaft hydrodynamic pump having opposing thrust-bearing axial hydrodynamic pumping action.
2. Description of the Related Art
Continuing advances in computer data storage technology strongly motivate improvements in magnetic disk areal storage densities. Increased data storage densities require corresponding increases in sensor-to-disk positioning precision. The typical magnetic disk data store includes several magnetic disks spinning at high speed while suspended on a common spindle bearing assembly, which includes a spinning bearing sleeve supported by a stationary bearing journal. Typically in the art, sensing heads are positioned to read or write streams of data from or to concentric tracks on each of the spinning magnetic disk surfaces. The precise radial position of each sensing head is controlled by a head actuator assembly that rotates on an actuator pivot bearing responsive to an actuator positioning motor, which operates to move each head from one track to another.
As the disk spindle bearing journal and bearing sleeve spin relative to one another, a point on the spin axis may trace out a path or orbit. The wobbling motion of this spin axis includes synchronous and asynchronous components, referred to in the art as repetitive "runout" and non-repetitive "runout," respectively. Hydrodynamic spindle bearing designs are preferred in the disk drive art over the older ball-bearing spindle systems because the rolling elements in ball-bearing spindle systems produce relatively large non-repetitive runout arising from several causes, including imperfect race and ball geometries, surface defects, non-axisymmetric radial stiffness, misalignments and imbalances. Modern servo tracking systems can compensate for most of the effects of repetitive spindle bearing runout. Uncompensated spindle bearing runout limits the available data storage density, which can be improved only by reducing and/or improving compensation for any spindle bearing assembly runout.
The track width and lineal data density determines the overall areal storage capacity of the disk surface. Any non-repetitive (e.g., vibration-induced) wobble in either the high-speed spindle bearing or the intermittently-rotating actuator pivot bearing affects the precise location of microscopic data storage sites on the disk surface with respect to the data sensing head. The available sensor-to-data repositioning precision imposes an upper limit on the overall areal storage capacity of the disk surface. Accordingly, vibration effects must be reduced in both the disk spindle bearing and the actuator pivot bearing to permit improved magnetic disk areal storage densities.
The actuator pivot bearing is also subject to "limit cycling" and "sticking" problems arising from the non-zero static bearing friction. Every time the actuator assembly is repositioned, the actuator motor must first overcome the pivot bearing static friction to begin the move and then halt the actuator assembly motion at precisely the desired position. For very small movements, such as those needed to follow a slightly-eccentric servo track (while compensating for repetitive spindle runout), the actuator first sticks because of static friction and then overshoots because of the sudden drop in friction occurring upon movement, and finally settles on track only after an unwanted delay. When commanded to move very slightly, the actuator may simply "stick" and not move at all. This imposes a lower limit on the available precision of any actuator movement and thereby prevents the precise tracking of servo-tracks with small eccentricities. The static friction of the actuator pivot bearing must be reduced to permit more refined actuator repositioning (such as required to better compensate for the effects of repetitive spindle bearing runout).
The disk drive art is replete with improvements in hydrodynamic spindle bearings for supporting disk rotational velocities of 10,000 RPM and more. For instance, a gas bearing is one well-known way of supporting a rapidly rotating shaft for some applications. Gas bearings may use either hydrostatic or hydrodynamic principles of operation. In a hydrostatic gas bearing, pressurized gas is supplied from an external source into the space between a rotating shaft and its surrounding sleeve. The gas acts as a lubricant and allows the shaft to rotate without coming into contact with the sleeve. In a hydrodynamic bearing, oblique grooves are cut in a shaft and the rotation of the shaft causes gas to flow through the grooves. The dynamic pressure created by this gas flow allows the gas to act as a lubricant during high-speed rotation of the shaft, thereby avoiding the need for an external mechanical compressor; such compressors are often undesirable because they occupy space within the device, consume power, and are a source of unwanted vibration and contamination.
In U.S. Pat. No. 5,328,270, Roy Crawford et al. disclose an integrated spindle bearing and pump assembly that includes a hydrodynamic pump disposed coaxially with respect to a hydrodynamic bearing. The integrated assembly provides pressurized fluid from one hydrodynamic bearing for use within the other hydrodynamic bearing. The pump bearing design is optimized for high flow pumping applications rather than the high pressure, zero flow requirements of the second support bearing, thereby optimizing the support bearing design without the need for an external compressor.
In U.S. Pat. No. 5,516,212, Forrest Titcomb discloses a spinning-shaft hydrodynamic spindle bearing having two or more radial and at least two axial thrust bearing layers with several improvements to control lubricating fluid pressure distribution to ensure balanced hydrostatic pressure throughout the entire bearing assembly in all of the several fluid bearing layers to prevent cavitation and lubricant blowout.
The hydrodynamic bearing that is so useful for spindle applications cannot be used to support head actuator assembly rotation because the actuator rotates back and forth intermittently over a small sector of perhaps less than sixty degrees and cannot sustain the continuous pumping action needed for a hydrodynamic bearing. Even so, in applications requiring very high precision (such as in factory servowriting machines) the hydrostatic gas bearing (with external gas compressor) has long been used to support head actuator assembly rotation. For instance, in U.S. Pat. No. 5,642,943, Wally Szeremeta discloses a self-aligning hydrostatic gas bearing particularly useful for supporting a distal end of an extended rotary apparatus in a system for the wholesale writing of servopatterns to the disk surfaces within a plurality of aligned head-disk assemblies. Such hydrostatic gas bearings are not suitable for use in production disk drive assemblies because of their size, weight, cost and the requirement for an external source of compressed gas.
Many practitioners in the disk drive art have proposed other improvements in head actuator pivot bearings intended to improve the precision of head-track positioning. For instance, in U.S. Pat. No. 5,666,242, John Edwards et al. disclose a pivot bearing assembly with an elastomeric interface disposed between the stationary member and the actuator to dampen any vibratory motion imparted to the head stack assembly. As illustrated by Edwards, et al., the usual practice is to use a ball-bearing assembly or a sealed journal bearing assembly together with various means for eliminating the transfer of shock and vibration to the head stack. Even so, the usual actuator pivot bearing assembly is a source of limit cycling, stickiness and external vibration transfer effects that limit available track-positioning precision, which burdens the servo tracking system and enforces an unwanted lower limit on track seek time.
It is desirable to resolve this problem by providing a useful hydrostatic bearing to support head actuator rotation. Until now, this has not been possible because of the well-known limitations discussed above. These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.